In the field of labelling colors to detect methamphetamine (MA), Japanese Unexamined Patent Publication No. HEI 3-223673 proposes an MA substituted with a dansyl group. This dansylized MA (DNS-MA) emits a 525 nm fluorescence intensity in a 330 nm excited state. The DNS-MA bonding to an MA-antibody changes its fluorescence intensity. When the DNS-MA encounters a newly added MA molecule, the DNS-MA leaves the antibody, reducing the intensity of 525 nm fluorescence. Observation of this change enables detection of even a very small amount of MA.
However, the detection capacity of such a method using DNS-MA is decreased by the presence of foreign substances. There are so many naturally-occurring substances having a fluorescence around 525 nm that it was difficult to detect MA from such a test sample contaminated with such substances. Further, such a method has not been able to detect cocaine because an appropriate labelling color for detecting cocaine by immunoassay was not developed.